There has been often experienced the blowing up of slag and metal at the initial stage of blowing of molten steel in a bottom blowing converter. The initial stage blowing up is a phenomenon, in which several 10 tons of slag and metal is bumped from the feed opening of a converter near the time of completion of desilication during the course of blowing in a bottom blowing converter. Particularly, the blowing up phenomenon has often occurred in case of the operation of a bottom blowing converter without lime powder injection. Therefore, it is eagerly demanded to prevent the initial stage blowing up in a stable operation of a top and bottom blowing converter, of which capacity of lime powder injection for the total amount of oxygen is considerably poorer than the bottom blowing converter.
While, it is commonly known that calcined lime is used in order to promote the dephosphorization of molten steel. It can be expected the same effect in the blowing of calcined fine lime powder into a top and bottom blowing converter through its bottom tuyere and in the blowing of lump-shaped lime into a top and bottom blowing converter through its feed opening similarly to the case of conventional top blowing converter. However, the reaction velocity in the top and bottom blowing converter is increased due to oxygen blown into the bottom of the converter and therefore there is a risk of initial stage blowing up before and after the completion of the above described desiliconization, and it is not advantageous to use lump-shaped lime.
Although the above described blowing of lime powder can decrease the above described risk, the risk can not be completely obviated. Therefore, a method for carrying out a stable operation in a top and bottom blowing converter is required.